Go to the Volume rendering area and adjust the Falloff to 2.6 so the edges are more defined, but it should now look kind of orange, so adjust the step size up to 2.13. Not only will this make the rendering faster but it will make it have less shading ect. so it's more bright and crisp.

Nearly done now but if you watch the animation the fire balls just keep exploding out, we want them to stay within it's explosion radius after they explode out. For this we need to use the drag spacewarp (available in max 4 only, if you're using max 3, go to Max3D and look for the Drag plugin from Mondo Media).

Drag the drag spacewarp out and assign it to the particle system. Go to the modify panel and make the linear X,Y,Z values all 5.0. This will tell the particle system to slow down after it emits.